Transmitters having active phased array antennas, i.e., phased array transmitters, are commonly used in the transmission of signals. Active phased array transmitters can be configured to transmit signals in highly directive beams by using multiple antenna elements each connected to an elementary transmitter power amplifier. The signals are fed through each power amplifier and are relatively phased to achieve constructive addition of radiation in the desired direction. When only a single signal is to be transmitted in a single beam, class-C single frequency power amplifiers may be efficiently utilized. The transmitted spectrum can then be determined from the phase modulation characteristics of the information modulated on the signal.
Phased array transmitters may also be configured to transmit multiple signals in multiple directed beams. In this case, combinations of the multiple signals to be transmitted are formed by a beamforming network using a set of beamforming coefficients. The signal combinations are fed to elementary transmit power amplifiers, with each power amplifier associated with a respective antenna element or subgroup of antenna elements. In this case, the power amplifiers must be linear, multiple signal power amplifiers, also known as Multiple Carrier Power Amplifiers or MCPA's. However, a problem with this prior art configuration is that imperfect MCPA linearity causes nonlinear distortion or intermodulation between the signals generating unwanted signals. These unwanted signals may lie outside the allocated frequency band, potentially interfering with other services. Even when each signal alone, and thus their linear combinations formed by a beamformer, are band limited, the non-linearly amplified combinations will exhibit out-of-band spectral components due to the non-linear distortion.
Improvements to such prior art active phased array transmitters for transmitting multiple signals in multiple beams while reducing intermodulation and improving efficiency are described in U.S. Pat. Nos. 5,548,813; 5,555,257; 5,568,088; 5,574,967; 5,594,941; 5,619,210; 5,619,503; and 5,638,024, all of which are herein incorporated by reference. The afore-listed patents are principally directed toward improving the efficiency of use of a phased array antenna and/or improving the intermodulation and efficiency performance of MCPA's used in phased array antennas. While the afore-listed patents are not principally directed toward reducing intermodulation radiation in unauthorized frequency bands, they may provide guidance.
While a single-signal phased array for transmitting a signal in a highly directive beam may of course be provided in multiple copies in order to transmit multiple signals, the total aperture area of the plurality of single-signal antenna arrays is inefficiently used since only a fraction of the antenna elements are used to radiate each signal. For instance, if two single-signal phased array antennas were used, each would use one-half of the total antenna elements; if four single-signal phased array antennas were used, each would use only one-fourth of the total antenna elements; etc. There is thus a need for a transmitter having an active phased array antenna which employs all antenna elements to radiate each directive beam in order to obtain the full directed gain of which the total antenna aperture area is capable, while avoiding excessive inter-modulation. Further, there is a need for a transmitter having an active phased array antenna useful in a situation where no MCPA of reasonable efficiency at the current state of the art can meet the stringent limits imposed on out-of-band intermodulation radiation, thus necessitating the use of single-carrier amplifiers while still allowing multiple signals to be radiated by the antenna element array.
The present invention is directed toward overcoming one or more of the above-mentioned problems.